This application pertains to fuses in general and more particularly to a microfuse and method of making microfuses using ultrasonic bonding.
Microfuses are used primarily in printed circuits and are required to be physically small. It is frequently necessary to provide fuses designed to interrupt surge currents in a very short period of time. For example, to limit potentially damaging surges in semiconductor devices, it is often necessary to interrupt 125 volt short circuit currents up to 50 amps AC or 300 amps DC in a time period of less than 0.001 seconds, in order to limit the energy delivered to the components in series with the fuse. Current art has interruption durations of approximately 0.008 seconds and i.sup.2 t values that could damage semiconductor devices.
Previous attempts to provide fuses operating in this range have utilized thin wires in air with a diameter of approximately 0.0005" to 0.015". The use of small diameter wire for fuse elements has a number of problems related to present manufacturing technology.
One problem is that it is difficult to manufacture a low-cost microfuse. The reason for this is that the fusible element has such a small diameter, measured in thousandths of an inch, that manual methods of attaching the fusible element to the lead wires or end caps is required.
Several problems are caused by use of solder and flux to attach the fusible wire element. In such a small device, it is difficult to prevent the solder used to attach the wire ends from migrating down the wire during the manufacturing process. This causes a change in the fuse rating. In addition, the fuse rating may be changed when the external leads are soldered onto a printed circuit board. Wave soldering, vapor phase soldering and other processes are typically used to solder parts to PC boards. The heat generated in these processes can melt and reflow the solder inside the fuse. Consequently, the fuse rating can be changed in the act of attaching the fuse of the PC board. It is also possible to lose contact to the fusible wire element entirely when the inner solder melts, rendering the fuse useless.
Another problem caused by the use of solder and flux inside the fuse body is that the solder and flux may be vaporized by the arc during a short circuit and can interfere with the arc interruption process.
An additional problem with present manufacturing processes is that it is difficult to accurately control the length of the wire element and to position it properly in the enclosing fuse body. Consequently, when hot, the wire element may contact the wall of the fuse body. This will also change the fuse rating and prevent the fuse from opening on low overloads.
Yet another problem with prior art design of microfuses is that the fusible element is not encapsulated in an arc quenching medium. The i.sup.2 t value for short circuit interruptions of wire elements in air is much greater as a consequence of the longer time required to achieve circuit interruption.